1. Field of the Invention
The present invention relates to a spindle motor for a hard disk drive device or the like provided with a fluid bearing and a method for manufacturing the same.
2. Description of the Related Art
In a spindle motor for driving a hard disk drive device (hereinafter referred to as HDD), there is a demand for rotation at high speed in accordance with a recent requirement of large capacity and high speed properties in a hard disk. A spindle motor in which a pivot of a rotor is supported by a fluid bearing having low sliding resistance and a high rotation precision is well known. FIG. 4 shows an example of a spindle motor provided with a fluid bearing 4 in which a substantially cylindrical stationary shaft 5 is positioned uprightly and coaxially with a pivot 61 of a rotor 3 in a stator 2, and also, a sleeve 7 into which the pivot 61 is to be inserted with a minute gap is fitted in a sleeve-fitting hole 5a formed in an inner circumferential surface of the stationary shaft 5. An opening on one side of the sleeve-fitting hole 5a is closed by a thrust plate 8 and a minute gap between the pivot 61 and the sleeve 7 and a space on the upper surface of the thrust plate 8 in communication with the minute gap are filled with a predetermined amount of fluid, and thus, the spindle motor, in which the pivot 61 is supported by the fluid bearing 4, is formed. As shown in FIG. 4, a hub 9 is fitted around the pivot 61 to form a rotor 3, and a plurality of hard disks 10 are mounted around an outer circumferential surface 9a of the hub 9, and magnets 11 are arranged at an equal spacing in an annular shape on an inner circumferential surface 9b of the hub 9. Also, an armature 12 including a stator stack 12a and a coil 12b is arranged to face the magnets 11 on the outer circumferential surface of the stationary shaft 5.
As shown in FIG. 4, in the spindle motor using such a fluid bearing, the thrust load from the rotor 3 is received by the stator 2 in a structure that the spherical pivot tip portion 61a is supported by the thrust plate 8, and an ideal rotation of the rotor 3 is created by a structure that the spherical portion of the pivot tip portion 61a contacts the thrust plate 8 at one point on the rotary shaft of the pivot 61. However, if the precision of the spherical surface of the pivot tip portion 61a is inferior, particularly in high speed rotation of the rotor 3, the contact point between the pivot tip portion 61a and the thrust plate 8 moves along the spherical surface of the pivot tip portion 61a causing vibration of the pivot 61, and thus causes the reading surface of the hard disk 10 to vibrate vertically.
However, forming a spherical surface at one end of a shaft involves a very difficult machining. The machining to form the tip portion 61a of the pivot 61 into a spherical surface with high precision has been an inefficient work demanding a lot of skill and time. Moreover, since the tip portion 61a of the pivot 61 rotates in contact with the thrust plate 8, it is preferable to perform a heat treat such as quenching; however, this was not possible due to a deformation on the high precision machined spherical surface caused by the heat treatment. Although it is possible to obtain the precision of the spherical surface by grinding the spherical surface after performing the heat treatment to the pivot tip portion 61a, it is very difficult and impractical to obtain high precision through grinding as it is well known. Lightweight and a remarkable cooling effect are expected by using a ceramic material for the thrust plate 8. In this case, however, it is not possible to subject the pivot tip portion 61a to a heat treatment, and thus the wearing resistance of the spherical face is insufficient. Therefore, usage of ceramic material on the thrust plate 8 had to be avoided. Also, it was a desire to facilitate the assembly and control of parts by integrating the pivot 61 with the hub 9. However, this could not be realized, since machining a spherical surface with high precision at one end of the shaft formed integrally with the hub 9 is extremely difficult.
Therefore, as shown in FIG. 5, conventionally used is a pivot 62 in which an internal conical portion 13 coaxial with the pivot 62 is formed in one side surface 62a of a shaft portion 62b of the pivot 62, and a steel ball 14 used in a bearing or the like that is superior in sphericity (coaxiality) and hardness is fixed or press-fitted in the internal conical portion 13. However, in a pivot formed like pivot 62, a diameter of the usable steel ball 14 is limited to a small level and it is impossible to form a large spherical surface having such a diameter that a higher rotational precision can be ensured. Since the pivot 62 still requires machining in the inner conical portion 13 with high precision on one side surface of the shaft, it is difficult to form the hub 9 and the pivot 62 integrally.
It is therefore an object of the present invention to provide a spindle motor and a manufacturing method therefor in which the spindle motor having a pivot which tip portion can easily be formed by a spherical surface having high precision, high hardness, which diameter can be determined as desired is manufactured.
In order to achieve the above object, according to a first aspect of the present invention, in a spindle motor comprising a rotor in which a pivot having a spherical surface at a tip portion rotates in unity with a hub, a stator having a substantially cylindrical stationary shaft uprightly and coaxially with the pivot, the stator having a sleeve abutting to the inner surface of the shaft, the pivot inserted into the sleeve within a minute gap, a thrust plate for closing an opening on one side of the stationary shaft and for supporting a spherical surface formed at the tip portion of the pivot, a fluid which is filled in the minute gap formed between the pivot and the sleeve and in a space above an upper face of the thrust plate which space communicating to the minute space between the pivot and the sleeve, the spindle motor is characterized in that the pivot has a tip portion formed by cross sectionally cutting a steel ball and fixing one end of the pivot to one side surface of the shaft.
With such a structure, it is possible to readily obtain the spherical tip portion of the pivot which diameter can be determined as desired having high precision and high hardness.
In the spindle motor according to a second aspect of the present invention, the thrust plate is formed of a ceramic material.
With such a structure, it is possible to reduce weight of the spindle motor and to enhance a cooling efficiency of the motor.
In the spindle motor according to a third aspect of the present invention, the pivot is formed integrally with the hub.
With such a structure, it is possible to reduce the parts and to facilitate the assembly and control of the parts.
According to a fourth aspect of the present invention, in a method for manufacturing a spindle motor comprising: a rotor in which a pivot having a spherical surface at a tip portion rotates in unity with a hub, a stator having a substantially cylindrical stationary shaft uprightly and coaxially with the pivot, the stator having a sleeve abutting to the inner surface of the shaft, the pivot inserted into the sleeve within a minute gap, a thrust plate for closing an opening on one side of the stationary shaft and for supporting a spherical surface formed at the tip portion of the pivot, a fluid which is filled in the minute gap formed between the pivot and the sleeve and in a space above an upper face of the thrust plate which space communicating to the minute space between the pivot and the sleeve, the method comprising a process for cross sectionally cutting a steel ball which forms a tip portion on the pivot, and a process of fixing one end of the pivot to one side surface of the shaft.
With such a structure the steel ball is cut in the steel ball cutting process, thereby it is possible to obtain the pivot tip portion formed by a spherical surface the diameter of which can be determined as desired having high precision and high hardness. Also, in the pivot tip portion bonding process, the pivot tip portion obtained by the steel ball cutting process is fixed to one side surface of the shaft, thereby it is possible to form the pivot with high rotation precision.